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Abstract Natural materials contain small grains of magnetic iron oxides that can record information about the magnetic field of the Earth when they form and can be used to document changes in the geomagnetic field through time. Thermoremanent magnetization is the most stable type of remanent magnetization in igneous rocks and can be carried by particle sizes above the upper size limit for single‐domain behavior. To better understand thermoremanent magnetization in particles larger than single domain, we imaged the thermal dependence of magnetic structures in ~1.5‐μm grains of titanomagnetite (Fe2.46Ti0.54O4) using variable‐temperature magnetic force microscopy. At room temperature, grains displayed single‐vortex and multivortex states. Upon heating, the single‐vortex state was found to be stable up to the Curie temperature (~215 °C), whereas multivortex states unblocked between 125 and 200 °C by transitioning into single‐vortex states. During cooling in a weak field (~0.1 mT), single‐vortex states nucleated just below the Curie temperature and remained unchanged to room temperature. The single‐vortex state was the only magnetic state observed at room temperature after weak field thermoremanent magnetization acquisition experiments. These observations indicate that single‐vortex states occur in titanomagnetite and, like single‐domain particles, have high thermal stability necessary for carrying stable paleomagnetic remanence.
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Thermomagnetic recording fidelity of nanometer-sized iron and implications for planetary magnetism
Paleomagnetic observations provide valuable evidence of the strength of magnetic fields present during evolution of the Solar System. Such information provides important constraints on physical processes responsible for rapid accretion of the protoplanetesimal disk. For this purpose, magnetic recordings must be stable and resist magnetic overprints from thermal events and viscous acquisition over many billions of years. A lack of comprehensive understanding of magnetic domain structures carrying remanence has, until now, prevented accurate estimates of the uncertainty of recording fidelity in almost all paleomagnetic samples. Recent computational advances allow detailed analysis of magnetic domain structures in iron particles as a function of grain morphology, size, and temperature. Our results show that uniformly magnetized equidimensional iron particles do not provide stable recordings, but instead larger grains containing single-vortex domain structures have very large remanences and high thermal stability—both increasing rapidly with grain size. We derive curves relating magnetic thermal and temporal stability demonstrating that cubes (>35 nm) and spheres (>55 nm) are likely capable of preserving magnetic recordings from the formation of the Solar System. Additionally, we model paleomagnetic demagnetization curves for a variety of grain size distributions and find that unless a sample is dominated by grains at the superparamagnetic size boundary, the majority of remanence will block at high temperatures ( ∼ 100 ° C of Curie point). We conclude that iron and kamacite (low Ni content FeNi) particles are almost ideal natural recorders, assuming that there is no chemical or magnetic alteration during sampling, storage, or laboratory measurement.
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- PAR ID:
- 10096630
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 116
- Issue:
- 6
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- 1984 to 1991
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Reliability of magnetic recordings of the ancient magnetic field is strongly dependent on the magnetic mineralogy of natural samples. Theoretical estimates of long‐term stability of remanence were restricted to single‐domain (SD) states, but micromagnetic models have recently demonstrated that the so‐called single‐vortex (SV) domain structure can have even higher stability that SD grains. In larger grains (10 μm in magnetite) the multidomain (MD) state dominates, so that large uniform magnetic domains are separated by narrow domain walls. In this paper we use a parallelized micromagnetic finite element model to provide resolutions of many millions of elements allowing us, for the first time, to examine the evolution of magnetic structure from a uniform state, through the SV state up to the development of the domain walls indicative of MD states. For a cuboctahedral grain of magnetite, we identify clear domain walls in grains as small as ∼3 μm with domain wall widths equal to that expected in large MD grains; we therefore put the SV to MD transition at ∼3 μm for magnetite and expect well‐defined, and stable, SV structures to be present until at least ∼1 μm when reducing the grain size. Reducing the size further shows critical dependence on the history of domain structures, particularly with SV states that transition through a so‐called “unstable zone” leading to the recently observed hard‐aligned SV states that proceed to unwind to SD yet remain hard aligned.more » « less
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SUMMARY The magnetic properties of iron-bearing minerals at above-ambient temperatures control their magnetic expression at depth in the Earth and other planets, as well as the permanent memory they retain as thermoremanence or thermochemical remanence when brought to the surface and cooled. This paper reports magnetic hysteresis parameters measured at temperatures up to the Curie point TC for natural pyrrhotite and hematite and for suites of sized magnetites, both natural and synthesized. Domain structure changes can be inferred from the ratio of saturation remanence Mrs to saturation magnetization Ms. In almost all magnetites and pyrrhotites studied, Mrs decreases more rapidly with increasing measurement temperature T than Ms, indicating thermal unblocking or vortex development in single-domain grains and addition or remobilization of domain walls at high T in multidomain grains. During cooling of a rock, iron minerals might then denucleate domains or vortices. Coercive force Hc, a measure of stability against changing magnetic fields, also decreases with increasing measurement T, usually at a rate similar to that of Mrs, but often retains a finite value near the Curie point.more » « less
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Abstract The ability of rocks to hold a reliable record of the ancient geomagnetic field depends on the structure and stability of magnetic domain‐states contained within constituent particles. In paleomagnetic studies, the Day plot is an easily constructed graph of magnetic hysteresis parameters that is frequently used to estimate the likely magnetic recording stability of samples. Often samples plot in the region of the Day plot attributed to so‐called pseudo‐single‐domain particles with little understanding of the implications for domain‐states or recording fidelity. Here we use micromagnetic models to explore the hysteresis parameters of magnetite particles with idealized prolate and oblate truncated‐octahedral geometries containing single domain (SD), single‐vortex and occasionally multi‐vortex states. We show that these domain states exhibit a well‐defined trend in the Day plot that extends from the SD region well into the multi‐domain region, all of which are likely to be stable remanence carriers. We suggest that although the interpretation of the Day plot and its variants might be subject to ambiguities, if the magnetic mineralogy is known, it can still provide some useful insights about paleomagnetic specimens' dominant domain state, average particle sizes and, consequently, their paleomagnetic stability.more » « less
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Abstract The minerals carrying the magnetic remanence in geological samples are commonly a solid solution series of iron‐titanium spinels known as titanomagnetites. Despite the range of possible compositions within this series, micromagnetic studies that characterize the magnetic domain structures present in these minerals have typically focused on magnetite. No studies systematically comparing the domain‐states present in titanomagnetites have been undertaken since the discovery of the single vortex (SV) structure and the advent of modern micromagnetism. The magnetic properties of the titanomagnetite series are known to vary strongly with composition, which may influence the domain states present in these minerals, and therefore the magnetic stability of the samples bearing them. We present results from micromagnetic simulations of titanomagnetite ellipsoids of varying shape and composition to find the size ranges of the single domain (SD) and SV structures. These size ranges overlap, allowing for regions where the SD and SV structures are both available. These regions are of interest as they may lead to magnetic instability and “partial thermal remanent magnetization (pTRM) tails” in paleointensity experiments. We find that although this SD + SV zone occupies a narrow range of sizes for equidimensional magnetite, it is widest for intermediate (TM30‐40) titanomagnetite compositions, and increases for both oblate and prolate particles, with some compositions and sizes having an SD + SV zone up to 100s of nm wide. Our results help to explain the prevalence of pTRM tail‐like behavior in paleointensity experiments. They also highlight regions of particles with unusual domain states to target for further investigation into the definitive mechanism behind paleointensity failure.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1073/pnas.1810797116
